During the past decade, rapid advances have been made in laser technology. These advances have led to the integration of lasers with other active optical elements such as detectors, optical amplifiers, optical modulators and switches on one semiconductor chip. Interconnections employed for this purpose are miniature, transparent, passive waveguides which pipe light from one device to the next and function in a manner similar to metals which carry electricity in conventional circuits. Further enhancements in this technology have been attained from advances in crystal growth and large area wafer processing.
Among the more recent developments in photonic integrated circuitry has been the corrugated-waveguide grating. The fabrication of these gratings became routine after the distributed feedback (DFB) laser was adopted worldwide as the preferred optical telecommunications source where high spectral purity is required. These gratings provide high quality on-chip resonators without encountering the restraints imposed by the usual cleaved-facet resonators employed in semiconductor lasers. Additionally, the gratings function as filters in certain receiver and amplifier applications.
Scanning electron micrographs in distributed feedback lasers reveal a corrugated interface between a higher index core material and a lower index cladding material. As a light beam propagates down such a waveguide, each bump reflects a small portion of the light. If the beam's wavelength is not close to the Bragg wavelength, all the reflections are out of phase and interfere destructively. At the Bragg wavelength, all reflections add in phase, so leading to a large cumulative reflection.
At the present time, the control of output wavelength of semiconductor lasers in distributed feedback (DFB) and tuning in distributed Bragg reflector (DBR) based lasers is achieved by means of the incorporation therein of the above-noted corrugated waveguide grating. In the DBR configuration, gratings of this type are typically installed by etching back a portion of the active layer and exposing the surface of an underlying passive waveguide. Alternatively, the grating may be installed in the substrate prior to the growth of the waveguide. In subsequent processing, an interference pattern formed by ultraviolet lasers is typically used to expose photoresist deposited on the exposed waveguide. The developed photoresist then forms the mask for etching the grating corrugation. The exposed portion of the structure is then regrown and contacts attached for operation. In the DFB configuration, the grating is usually installed prior to growth of the active layer or waveguide. This end is typically attained by using an interference pattern, as described above, to expose photoresist deposited on the exposed passive waveguide. The exposed photoresist forms a mask for etching the corrugation. The active layer is then grown over the grating and suitable contacts are added to effect operation.